Caused by: org.apache.beam.sdk.coders.CoderException: Cannot encode a null value.
at org.apache.beam.sdk.extensions.kryo.KryoCoder.encode(KryoCoder.java:197)
at cz.o2.proxima.beam.core.io.PairCoder.encode(PairCoder.java:59)
at cz.o2.proxima.beam.core.io.PairCoder.encode(PairCoder.java:31)
at org.apache.beam.sdk.coders.Coder.encode(Coder.java:136)
at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:590)
at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:581)
at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:541)
at org.apache.beam.runners.flink.translation.types.CoderTypeSerializer.serialize(CoderTypeSerializer.java:105)
at org.apache.flink.runtime.plugable.SerializationDelegate.write(SerializationDelegate.java:54)